1. Field of the Invention
This invention relates generally to ferromagnetic alloy thin-films and more particularly to alloy films with high saturation flux densities for magnetic write heads.
2. Description of the Related Art
The terminology and units used in the magnetic materials arts vary from one region to another. Accordingly, a brief summary of terminology used herein is presented for clarity. Magnetic Flux is expressed in Système International d'unités (SI) units of webers (Wb) or volt-seconds, each of which is exactly equivalent to 100,000,000 maxwells (Mx). Magnetic Flux Density (B) is expressed in SI units of teslas (T), each of which is exactly equivalent to 10,000 gauss (G). Magnetic Field Intensity (H) is expressed in SI units of amperes per meter (A/m), each of which is approximately equivalent to about 0.0126 oersteds (Oe). As used herein, the Permanent Magnetic Moment or Magnetization (BM) of a material is the magnetic flux density (B) in teslas present in the material with no external magnetic H-field applied. The Saturation Flux Density (BS) of a material (commonly denominated 4πMS) is the maximum magnetic flux density (B) in teslas that can be induced in the material by a large external magnetic field (H-field). The Remanence or Retentivity (BR<BS) of a material is the magnetic moment in teslas remaining in the material after forcing the material into saturation along the easy axis and then removing the external H-field. The Coercivity (HC) of a material is the magnetic field (H-field) intensity in amperes per meter required to overcome the remanence moment (BR) to reduce the magnetic flux density (B) in the material to zero along the easy axis. The Anisotropic Field (HK>HC) of a material (also may be denominated “the magnetic anisotropy”) is the magnetic field (H-field) intensity in amperes per meter required to induce the saturation flux density (BS) in the material along the hard axis normal to the easy axis. The permeability of a material (μ) is defined as the ratio B/H with appropriate units and may be shown to be about the same as BS/HK when large. The permeability of free space is defined such that a magnetic field intensity (H) of one oersted produces a magnetic flux density (B) of one gauss. Accordingly, a large external H-field may be applied to force a material into saturation along the easy axis and induce therein the maximum magnetic flux density (BS) possible for the material. Removing the external H-field leaves a permanent remanence moment (BR) in the material oriented along the easy axis. A reverse external H-field equal to the material coercivity (HC) may then be applied along the easy axis to reduce the magnetic flux density (B) in the material to zero.
The thin-film inductive head and the inductive/magnetoresistive (MR) head are well-known in the art. Both of these heads can write and read signals with respect to a magnetic medium such as a rotating disk medium or a streaming tape medium. The inductive head usually includes first and second poles having first and second ferromagnetic (FM) pole tips, respectively. The pole tips are separated by a gap at an air bearing surface (ABS) or head surface. A coil is disposed between the first and second poles to couple magnetically thereto. The MR head uses an inductive write head portion to perform write functions and a MR read head portion to perform read functions. The read head portion includes an MR sensor sandwiched between a pair of read gap layers, which are in turn sandwiched between first and second shield layers. Either type of magnetic head is usually mounted on or embedded in a slider that is supported in a transducing relationship with respect to a magnetic medium. The magnetic medium may be either a magnetic disk or a magnetic tape.
The pole pieces, including the pole tips, are commonly constructed of Permalloy (Ni81Fe19), which combines 81% nickel with 19% iron by weight. Permalloy is a desirable material for pole-construction, having good soft magnetic properties (low coercivity and high saturation flux density) and being easy to shape by normal patterning and deposition techniques. Further, Permalloy has good corrosion resistance for head reliability. Permalloy has a saturation flux density (BS) of about 1.0 T (10 kG) and a coercivity (HC) of no more than 20 A/m (0.2 Oe) at worst. But it is desirable to increase the saturation flux density (BS) well above this value so that the pole tips can carry the larger magnetic flux density required to overcome the high coercivity of modern high-density magnetic data storage media.
Cobalt-based magnetic alloys have a higher saturation flux density (BS) than does Permalloy. However, cobalt materials have significantly worse corrosion resistance. Another family of high-BS materials is the sputtered FeNiX materials, where X is from the group of tantalum, aluminum, and rhodium. But sputter-deposition of the pole pieces is not as desirable as frame-plating because ion-milling is required after sputtering to shape the trackwidth of the pole tips. This process is very difficult to implement. And sputtered materials exhibit a high stress that can distort recorded signals. Moreover, magnetically forming a thick film of such materials using sputtering is difficult because the sputtered material has a large magnetocrystalline anisotropy and the crystal grain size of the sputtered film becomes large so the anisotropic field (HK) is disadvantageously large. An electroplating method is preferred to suppress the crystal grain size to a small value to reduce the anisotropic field (HK) while retaining the desired high saturation flux density (BS); this effectively increases the permeability (μ≈BS/HK) of the thin-film material.
For example, the commonly-assigned U.S. Pat. No. 4,661,216 discloses an electroplating bath composition useful for fabricating thin-film layers of magnetic cobalt-nickel-iron alloys with high percentages of cobalt. With the disclosed method, pole-piece layers may be fabricated with saturation flux densities (BS) of over 1.4 T (14 kG) while coercivity (HC) is less than 160 A/m (2 Oe). However, these values are not sufficient to fully exploit the capacity of modern high-density data storage media.
Considerable more recent effort has been undertaken by practitioners in the art to increase the recording density of magnetic heads. Decreasing the length (i.e., the thickness) of the gap between the first and second pole tips permits writing of more bits per inch of media. Further, increasing the coercivity (HC) of the magnetic medium allows the medium to accurately retain data with a higher areal bit density with less thermal degradation. A consequence of such higher bit density is a higher data transfer rate for information between the head and the medium. These magnetic media coercivity and density improvements require the magnetic pole materials to conduct relatively high magnetic flux densities, especially those portions of the poles (the pole tips) adjacent to the gap at the ABS. However, the ferromagnetic (FM) pole materials have a saturation flux density (BS) limit beyond which they can conduct no more magnetic flux. Accordingly, there is still a clearly-felt need for a pole tip structure having a high saturation flux density (BS) to operate effectively with newer high-coercivity magnetic media.
For example, the U.S. Pat. No. 5,763,108 discloses an electroplating method for forming thin films of a nickel-iron alloy having (preferably) from 54% to 56% of iron by weight alloyed with less than 0.5% tin by weight. Bath temperature is maintained above 20° C. to about 35° C. and includes from about 0.4 to about 0.9 moles per liter of iron (Fe++) ions. Annealing is accomplished at from 120° C. to 300° C. A disclosed process for simultaneously thermally annealing and anisotropically magnetically aligning the pole-piece layers results in a saturation flux density (BS) of as high as 1.6 T (16 kG), a coercivity (HC) of less than 80 A/m (1 Oe) and an anisotropic field (HK) of 650 to 1300 A/m (8 to 16 Oe). However, these values are not sufficient to fully exploit the capacity of modern high-density data storage media.
As another example, the U.S. Pat. No. 6,118,628 describes an electroplating method for fabricating thin films using a nickel-iron alloy having up to 62% iron by weight and adding less than 15% cobalt or less than 3% of molybdenum, chromium, boron, indium, palladium or the like. Electroplating bath temperature is maintained above 20° C. to about 35° C. and the nickel-to-iron ion ratio (Ni++/Fe++) is maintained between 7 and 8. Annealing is accomplished at from 120° C. to 300° C. Saturation flux densities from 1.3 to 1.65 T (13 to 16.5 kG) were obtained while limiting coercivity to less than 80 A/m (1 Oe by electroplating a pole layer through a mask using one of these third elements in a nickel-iron alloy with about 55% iron by weight. However, these values are not sufficient to fully exploit the capacity of modem high-density data storage media.
The pole material of choice in the art is currently a nickel-iron alloy with from 50% to 60% iron by weight (often denominated Ni45Fe55), the properties of which have not yet been substantially improved by the addition of minor portions of other elements. But the saturation flux density of this material is limited to about 1.75 T (17.5 kG) at best, even with other trace elements, perhaps because of inclusions of oxides and other unwanted impurities during the electroplating processes known in the art. It would be desirable to improve the saturation flux density (BS) of this nickel-iron alloy by adding more iron to provide more than 62% iron by weight but the art generally teaches away from this proposal because FM alloys with higher iron concentrations are expected to have a coercivity (HC) of over 250 A/m (over 3 Oe), which is too high to handle the high frequencies required to write high-density data to a high-coercivity medium. So a useful method for creating nickel-iron alloy thin films exhibiting high saturation magnetization and low coercivity is very desirable but, until now, has been unknown in the art. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.